Progressive locomotor recalibration during blind walking

Visual Perceptual Confidence: Exploring Discrepancies Between Self-reported and Actual Distance Perception In Virtual Reality

Virtual Reality (VR) systems are widely used, and it is essential to know if spatial perception in virtual environments (VEs) is similar to reality. Research indicates that users tend to underestimate distances in VR. Prior work suggests that actual distance judgments in VR may not always match the users self-reported preference of where they think they most accurately estimated distances. However, no explicit investigation evaluated whether user preferences match actual performance in a spatial judgment task. We used blind walking to explore potential dissimilarities between actual distance estimates and user-selected preferences of visual complexities, VE conditions, and targets. Our findings show a gap between user preferences and actual performance when visual complexities were varied, which has implications for better visual perception understanding, VR applications design, and research in spatial perception, indicating the need to calibrate and align user preferences and true spatial perception abilities in VR.

Assessing the relative contribution of vision to odometry via manipulations of gait in an over-ground homing task

The visual, vestibular, and haptic perceptual systems are each able to detect self-motion. Such information can be integrated during locomotion to perceive traversed distances. The process of distance integration is referred to as odometry. Visual odometry relies on information in optic flow patterns. For haptic odometry, such information is associated with leg movement patterns. Recently, it has been shown that haptic odometry is differently calibrated for different types of gaits. Here, we use this fact to examine the relative contributions of the perceptual systems to odometry. We studied a simple homing task in which participants travelled set distances away from an initial starting location (outbound phase), before turning and attempting to walk back to that location (inbound phase). We manipulated whether outbound gait was a walk or a gallop-walk. We also manipulated the outbound availability of optic flow. Inbound reports were performed via walking with eyes closed. Consistent with previous studies of haptic odometry, inbound reports were shorter when the outbound gait was a gallop-walk. We showed that the availability of optic flow decreased this effect. In contrast, the availability of optic flow did not have an observable effect when the outbound gait was walking. We interpreted this to suggest that visual odometry and haptic odometry via walking are similarly calibrated. By measuring the decrease in shortening in the gallop-walk condition, and scaling it relative to the walk condition, we estimated a relative contribution of optic flow to odometry of 41%. Our results present a proof of concept for a new, potentially more generalizable, method for examining the contributions of different perceptual systems to odometry, and by extension, path integration. We discuss implications for understanding human wayfinding.

Egocentric Distance Perception: A Comparative Study Investigating Differences Between Real and Virtual Environments

Virtual reality systems are a popular tool in behavioral sciences. The participants' behavior is, however, a response to cognitively processed stimuli. Consequently, researchers must ensure that virtually perceived stimuli resemble those present in the real world to ensure the ecological validity of collected findings. Our article provides a literature review relating to distance perception in virtual reality. Furthermore, we present a new study that compares verbal distance estimates within real and virtual environments. The virtual space-a replica of a real outdoor area-was displayed using a state-of-the-art head-mounted display. Investigated distances ranged from 8 to 13 m. Overall, the results show no significant difference between egocentric distance estimates in real and virtual environments. However, a more in-depth analysis suggests that the order in which participants were exposed to the two environments may affect the outcome. Furthermore, the study suggests that a rising experience of immersion leads to an alignment of the estimated virtual distances with the real ones. The results also show that the discrepancy between estimates of real and virtual distances increases with the incongruity between virtual and actual eye heights, demonstrating the importance of an accurately set virtual eye height.

Iterative Spatial Updating During Forward Linear Walking Revealed Using a Continuous Pointing Task

The continuous pointing task uses target-directed pointing responses to determine how perceived distance traveled is estimated during forward linear walking movements. To more precisely examine the regulation of this online process, the current study measured upper extremity joint angles and step-cycle kinematics in full vision and no-vision continuous pointing movements. Results show perceptual under-estimation of traveled distance in no-vision trials compared to full vision trials. Additionally, parsing of the shoulder plane of elevation trajectories revealed discontinuities that reflected this perceptual under-estimation and that were most frequently coupled with the early portion of the right foot swing phase of the step-cycle. This suggests that spatial updating may be composed of discrete iterations that are associated with gait parameters.

The role of top-down knowledge about environmental context in egocentric distance judgments

Judgments of egocentric distances in well-lit natural environments can differ substantially in indoor versus outdoor contexts. Visual cues (e.g., linear perspective, texture gradients) no doubt play a strong role in context-dependent judgments when cues are abundant. Here we investigated a possible top-down influence on distance judgments that might play a unique role under conditions of perceptual uncertainty: assumptions or knowledge that one is indoors or outdoors. We presented targets in a large outdoor field and in an indoor classroom. To control visual distance and depth cues between the environments, we restricted the field of view by using a 14-deg aperture. Evidence of context effects depended on the response mode: Blindfolded-walking responses were systematically shorter indoors than outdoors, whereas verbal and size gesture judgments showed no context effects. These results suggest that top-down knowledge about the environmental context does not strongly influence visually perceived egocentric distance. However, this knowledge can operate as an output-level bias, such that blindfolded-walking responses are shorter when observers' top-down knowledge indicates that they are indoors and when the size of the room is uncertain.

Effect of Transient Perturbations of Short-Term Memory on Target-Directed Blind Locomotion

It is difficult to walk without vision to a nearby destination if there is a time delay between watching the destination and walking toward it. Indeed, path deviation occurred when delays were introduced before initiating straight ahead blindfolded walking (R. A. Tyrrell, K. K., Rudolph, B. G., Eggers, & H. W. Leibowitz, 1993 ). The questions are whether the location of a 60-s delay in the walking path and whether performing a cognitive task during the delay influence the accuracy in reaching a previously seen target while walking without vision. Thirty young adults walked blindfolded and stopped when they believed they had reached a target at 8 m. Delays were 60 s in duration, were located at 0, 4, and 7 m, and involved waiting or backward counting. Significant differences were found between 0-m and 4-m delay locations for distance to target, distance travelled and path deviation (p < .05). Significant effect of backward counting during the 60-s delay was found at the 0-m delay for distance travelled (p < .05). The interaction between retaining visual guidance information during 60 s and performing a cognitive task likely influenced target-directed blind navigation.

The Roles for Prior Visual Experience and Age on the Extraction of Egocentric Distance

OBJECTIVES

In a well-lit room, observers can generate well-constrained estimates of the distance to an object on the floor even with just a fleeting glimpse. Performance under these conditions is typically characterized by some underestimation but improves when observers have previewed the room. Such evidence suggests that information extracted from longer durations may be stored to contribute to the perception of distance at limited time frames. Here, we examined the possibility that this stored information is used differentially across age. Specifically, we posited that older adults would rely more than younger adults on information gathered and stored at longer glimpses to judge the distance of briefly glimpsed objects.

METHOD

We collected distance judgments from younger and older adults after brief target glimpses. Half of the participants were provided 20-s previews of the testing room in advance; the other half received no preview.

RESULTS

Performance benefits were observed for all individuals with prior visual experience, and these were moderately more pronounced for the older adults.

DISCUSSION

The results suggest that observers store contextual information gained from longer viewing durations to aid in the perception of distance at brief glimpses, and that this memory becomes more important with age.

To Hear or Not to Hear: Sound Availability Modulates Sensory-Motor Integration

When we walk in place with our eyes closed after a few minutes of walking on a treadmill, we experience an unintentional forward body displacement (drift), called the sensory-motor aftereffect. Initially, this effect was thought to be due to the mismatch experienced during treadmill walking between the visual (absence of optic flow signaling body steadiness) and proprioceptive (muscle spindles firing signaling body displacement) information. Recently, the persistence of this effect has been shown even in the absence of vision, suggesting that other information, such as the sound of steps, could play a role. To test this hypothesis, six cochlear-implanted individuals were recruited and their forward drift was measured before (Control phase) and after (Post Exercise phase) walking on a treadmill while having their cochlear system turned on and turned off. The relevance in testing cochlear-implanted individuals was that when their system is turned off, they perceive total silence, even eliminating the sounds normally obtained from bone conduction. Results showed the absence of the aftereffect when the system was turned off, underlining the fundamental role played by sounds in the control of action and breaking new ground in the use of interactive sound feedback in motor learning and motor development.

The brain's sense of walking: a study on the intertwine between locomotor imagery and internal locomotor models in healthy adults, typically developing children and children with cerebral palsy

Motor imagery and internal motor models have been deeply investigated in literature. It is well known that the development of motor imagery occurs during adolescence and it is limited in people affected by cerebral palsy. However, the roles of motor imagery and internal models in locomotion as well as their intertwine received poor attention. In this study we compared the performances of healthy adults (n = 8, 28.1 ± 5.1 years old), children with typical development (n = 8, 8.1 ± 3.8 years old) and children with cerebral palsy (CCP) (n = 12, 7.5 ± 2.9 years old), measured by an optoelectronic system and a trunk-mounted wireless inertial magnetic unit, during three different tasks. Subjects were asked to achieve a target located at 2 or 3 m in front of them simulating their walking by stepping in place, or actually walking blindfolded or normally walking with open eyes. Adults performed a not significantly different number of steps (p = 0.761) spending not significantly different time between tasks (p = 0.156). Children with typical development showed task-dependent differences both in terms of number of steps (p = 0.046) and movement time (p = 0.002). However, their performance in simulated and blindfolded walking (BW) were strictly correlated (R = 0.871 for steps, R = 0.673 for time). Further, their error in BW was in mean only of -2.2% of distance. Also CCP showed significant differences in number of steps (p = 0.022) and time (p < 0.001), but neither their number of steps nor their movement time recorded during simulated walking (SW) were found correlated with those of blindfolded and normal walking (NW). Adults used a unique strategy among different tasks. Children with typical development seemed to be less reliable on their motor predictions, using a task-dependent strategy probably more reliable on sensorial feedback. CCP showed less efficient performances, especially in SW, suggesting an altered locomotor imagery.

Pointing to azimuths and elevations of targets: blind and blindfolded-sighted

Three groups of observers pointed to target circles in a path on the ground, in two parallel rows. Participants in one group viewed the circles and then pointed blindfolded. Those in a second group were blindfolded and then touched the circles with a stick while walking past them. Volunteers in the third group were blind adults, a diverse group, who also used a stick to detect the circles. For all three groups, as distance to the circles increased, pointing azimuths shrank and elevations increased. We suggest that directions to targets on major environmental surfaces may be appreciated similarly by the blind and sighted. We challenge the assumption that the principle of convergence to the horizon, available through vision because of the way in which visual angle decreases on the retina, is not available through touch.

Medial temporal lobe roles in human path integration

Path integration is a process in which observers derive their location by integrating self-motion signals along their locomotion trajectory. Although the medial temporal lobe (MTL) is thought to take part in path integration, the scope of its role for path integration remains unclear. To address this issue, we administered a variety of tasks involving path integration and other related processes to a group of neurosurgical patients whose MTL was unilaterally resected as therapy for epilepsy. These patients were unimpaired relative to neurologically intact controls in many tasks that required integration of various kinds of sensory self-motion information. However, the same patients (especially those who had lesions in the right hemisphere) walked farther than the controls when attempting to walk without vision to a previewed target. Importantly, this task was unique in our test battery in that it allowed participants to form a mental representation of the target location and anticipate their upcoming walking trajectory before they began moving. Thus, these results put forth a new idea that the role of MTL structures for human path integration may stem from their participation in predicting the consequences of one's locomotor actions. The strengths of this new theoretical viewpoint are discussed.

Angular declination and the dynamic perception of egocentric distance

The extraction of the distance between an object and an observer is fast when angular declination is informative, as it is with targets placed on the ground. To what extent does angular declination drive performance when viewing time is limited? Participants judged target distances in a real-world environment with viewing durations ranging from 36-220 ms. An important role for angular declination was supported by experiments showing that the cue provides information about egocentric distance even on the very first glimpse, and that it supports a sensitive response to distance in the absence of other useful cues. Performance was better at 220-ms viewing durations than for briefer glimpses, suggesting that the perception of distance is dynamic even within the time frame of a typical eye fixation. Critically, performance in limited viewing trials was better when preceded by a 15-s preview of the room without a designated target. The results indicate that the perception of distance is powerfully shaped by memory from prior visual experience with the scene. A theoretical framework for the dynamic perception of distance is presented.

The visual system's intrinsic bias influences space perception in the impoverished environment

A dimly lit target in the intermediate distance in the dark is judged at the intersection between the target's projection line from the eye to its physical location and an implicit slanted surface, which is the visual system's intrinsic bias. We hypothesize that the intrinsic bias also contributes to perceptual space in the impoverished environment. We first showed that a target viewed against sparse texture elements delineating the horizontal ground surface in the dark is localized along an implicit slanted surface that is less slanted than that of the intrinsic bias, reflecting the weighted integration of the weak texture information and intrinsic bias. We also showed that while the judged egocentric locations are similar between 0.15- to 5-s exposure durations, the judged precision improves with duration. Furthermore, the precision for the judged target angular declination does not vary with the physical angular declination and is better than the precision of the eye-to-target distance. Second, we used both action and perceptual tasks to directly reveal the perceived surface slant. Confirming our hypothesis, we found that an L-shaped target on the horizontal ground with sparse texture information is perceived with a slant that is less than that of the intrinsic bias.

The visual system's intrinsic bias and knowledge of size mediate perceived size and location in the dark

Dimly lit targets in the dark are perceived as located about an implicit slanted surface that delineates the visual system's intrinsic bias (Ooi, Wu, & He, 2001). If the intrinsic bias reflects the internal model of visual space-as proposed here-its influence should extend beyond target localization. Our first 2 experiments demonstrated that the intrinsic bias also influences perceived target size. We employed a size-matching task and an action task to measure the perceived size of a dimly lit target at various locations in the dark. Then using the size distance invariance hypothesis along with the accurately perceived target angular declination, we converted the perceived sizes to locations. We found that the derived locations from the size judgment tasks can be fitted by slanted curves that resemble the intrinsic bias profile from judged target locations. Our third experiment revealed that armed with the explicit knowledge of target size, an observer perceives target locations in the dark following an intrinsic bias-like profile that is shifted slightly farther from the observer than the profile obtained without knowledge of target size (i.e., slightly more veridical). Altogether, we showed that the intrinsic bias serves as an internal model, or memory, of ground surface layouts when the visual system cannot rely on external depth information. This memory/model can also be weakly influenced by top-down knowledge.

Peripheral stimulation and its effect on perceived spatial scale in virtual environments

The following series of experiments explore the effect of static peripheral stimulation on the perception of distance and spatial scale in a typical head-mounted virtual environment. It was found that applying constant white light in an observer's far periphery enabled the observer to more accurately judge distances using blind walking. An effect of similar magnitude was also found when observers estimated the size of a virtual space using a visual scale task. The presence of the effect across multiple psychophysical tasks provided confidence that a perceptual change was, in fact, being invoked by the addition of the peripheral stimulation. These results were also compared to observer performance in a very large field of view virtual environment and in the real world. The subsequent findings raise the possibility that distance judgments in virtual environments might be considerably more similar to those in the real world than previous work has suggested.

Does perceptual-motor calibration generalize across two different forms of locomotion? Investigations of walking and wheelchairs

The relationship between biomechanical action and perception of self-motion during walking is typically consistent and well-learned but also adaptable. This perceptual-motor coupling can be recalibrated by creating a mismatch between the visual information for self-motion and walking speed. Perceptual-motor recalibration of locomotion has been demonstrated through effects on subsequent walking without vision, showing that learned perceptual-motor coupling influences a dynamic representation of one's spatial position during walking. Our present studies test whether recalibration of wheelchair locomotion, a novel form of locomotion for typically walking individuals, similarly influences subsequent wheelchair locomotion. Furthermore, we test whether adaptation to the pairing of visual information for self-motion during one form of locomotion transfers to a different locomotion modality. We find strong effects of perceptual-motor recalibration for matched locomotion modalities--walking/walking and wheeling/wheeling. Transfer across incongruent locomotion modalities showed weak recalibration effects. The results have implications both for theories of perceptual-motor calibration mechanisms and their effects on spatial orientation, as well as for practical applications in training and rehabilitation.

Age differences in virtual environment and real world path integration

Accurate path integration (PI) requires the integration of visual, proprioceptive, and vestibular self-motion cues and age effects associated with alterations in processing information from these systems may contribute to declines in PI abilities. The present study investigated age-related differences in PI in conditions that varied as a function of available sources of sensory information. Twenty-two healthy, young (23.8 ± 3.0 years) and 16 older (70.1 ± 6.4 years) adults participated in distance reproduction and triangle completion tasks (TCTs) performed in a virtual environment (VE) and two “real world” conditions: guided walking and wheelchair propulsion. For walking and wheelchair propulsion conditions, participants wore a blindfold and wore noise-blocking headphones and were guided through the workspace by the experimenter. For the VE condition, participants viewed self-motion information on a computer monitor and used a joystick to navigate through the environment. For TCTs, older compared to younger individuals showed greater errors in rotation estimations performed in the wheelchair condition, and for rotation and distance estimations in the VE condition. Distance reproduction tasks (DRTs), in contrast, did not show any age effects. These findings demonstrate that age differences in PI vary as a function of the available sources of information and by the complexity of outbound pathway.

Effects of visual deprivation on gait dynamic stability

Vision can improve bipedal upright stability during standing and affect spatiotemporal parameters during walking. However, little is known about the effects of visual deprivation on gait dynamic stability. We have tested 28 subjects during walking under two different visual conditions, full vision (FV) and no vision (NV), measuring their upper body accelerations. Lower accelerations were found in NV for the reduced walking speed. However, the normalized accelerations were higher in the NV than in the FV condition, both in anteroposterior (1.05 ± 0.21 versus 0.88 ± 0.16, P = 0.001) and laterolateral (0.99 ± 0.26 versus 0.78 ± 0.19, P < 0.001) directions. Vision also affected the gait anteroposterior harmony (P = 0.026) and, interacting with the environment, also the latero-lateral one (P = 0.017). Directly (as main factor of the ANOVA) or indirectly (by means of significant interactions with other factors), vision affected all the measured parameters. In conclusion, participants showed an environment-dependent reduction of upper body stability and harmony when deprived by visual feedback.

Walking there: environmental influence on walking-distance estimation

In a dark environment, when vision is excluded, humans are usually able to walk towards a target the position of which was previously memorized. Changes in spatio-temporal gait parameters, the presence of obstacles on the ground or pathway tilt can affect their performances. The aim of this study was to investigate the influence of the environment on this ability. We have enrolled sixty healthy subjects, separately tested in a small indoor and in an outdoor open-field environment. In experiment 1, significant differences were found between 15 indoor and 15 outdoor blindfolded walkers. According to previous studies, the distances walked outdoors were not significantly different from the three-tested target's distances (3m, 6m and 10m). Conversely, a systematic and significant undershooting was observed for blindfolded indoor walkers for all the three distances (errors: -0.34, -0.73 and -1.99m, respectively). This indoor undershooting was found related to shorter steps not compensated by any increment of the step number. In experiment 2, also the perception of the indoor distance resulted underestimated in other two tested groups of 15 subjects each. But the perceived distance resulted poorly correlated with motor performances (R=0.23, p=0.410). In spite of the fact that the errors were consistent among trials, when indoor walkers could not access to environmental acoustic features, their performance resulted highly variable among subjects, but it improved, on average. At the light of these results, the environment seems acting as a selective tuning between different strategies.

Treadmill experience mediates the perceptual-motor aftereffect of treadmill walking

People have a lifetime of experience in which to calibrate their self-produced locomotion with the resultant optical flow. Contrary to walking across the ground, however, walking on a treadmill produces minimal optical flow, and consequentially, a perceptual-motor aftereffect results. We demonstrate that the magnitude of this perceptual-motor aftereffect-measured by forward drift while attempting to march in-place following treadmill walking-decreases as experience walking on a treadmill is acquired over time. Experience with treadmill walking enables walking in this context to become sufficiently distinguished from walking in other contexts. Consequently, two distinct perceptual-motor calibration states are maintained, each linked to the context in which walking occurs. Experience with treadmill walking maintains perceptual-motor calibration accuracy in both walking contexts, despite changes to the relationship between perception and action.

The underestimation of egocentric distance: evidence from frontal matching tasks

There is controversy over the existence, nature, and cause of error in egocentric distance judgments. One proposal is that the systematic biases often found in explicit judgments of egocentric distance along the ground may be related to recently observed biases in the perceived declination of gaze (Durgin & Li, Attention, Perception, & Psychophysics, in press), To measure perceived egocentric distance nonverbally, observers in a field were asked to position themselves so that their distance from one of two experimenters was equal to the frontal distance between the experimenters. Observers placed themselves too far away, consistent with egocentric distance underestimation. A similar experiment was conducted with vertical frontal extents. Both experiments were replicated in panoramic virtual reality. Perceived egocentric distance was quantitatively consistent with angular bias in perceived gaze declination (1.5 gain). Finally, an exocentric distance-matching task was contrasted with a variant of the egocentric matching task. The egocentric matching data approximate a constant compression of perceived egocentric distance with a power function exponent of nearly 1; exocentric matches had an exponent of about 0.67. The divergent pattern between egocentric and exocentric matches suggests that they depend on different visual cues.